1. Field of the Invention
The present invention relates to an optical pickup and optical recording and/or reproducing apparatus designed to optimally realize a tracking servo for various kinds of optical discs by allowing compatibility between the discs, and a method of realizing compatible tracking.
2. Description of the Related Art
In general, a differential push-pull (DPP) method exhibiting excellent shift characteristics of an objective lens is mostly used in tracking of an optical pickup for recording. The DPP method uses zero-order and plus and minus first-order beams diffracted by a grating where a phase difference between plus and minus first-order diffracted beams illuminated on an optical disc is 180°.
FIG. 1 schematically shows the configuration of a photodetector 10 used in an optical pickup designed to realize a tracking servo using a DPP method. The photodetector 10 having eight sections is comprised of a main photodetector 11 divided into four sections A, B, C, and D and a pair of sub photodetectors 13 and 15, each being divided into two sections I1 and I2, and J1 and J2, respectively. While a zero-order beam diffracted by a grating is received by the main photodetector 11, plus and minus first-order diffracted beams are received by the sub photodetectors 13 and 15, respectively.
In this case, a tracking error signal is detected by a DPP method by taking the difference between the sum of detection signals from sections I1 and J1 of sub photodetectors 13 and 15 and the sum of detection signals from the remaining sections I2 and J2.
There is no problem with implementing a tracking servo for either DVD±R/RW or DVD-RAM using a typical DPP method. However, where an optical pickup designed to realize a tracking servo by DPP is employed for DVD±R/RW and DVD-RAM, the tracking servo is impossible to properly implement since the track pitch difference between the DVD±R/RW and DVD-RAM results in an extremely large phase difference between the plus and minus first-order beams.
Thus, if a phase is precisely fitted with respect to either DVD±R/RW or DVD-RAM, then the first-order beams are completely out of phase with respect to the other, which makes tracking impossible to implement. Furthermore, as evident from a graph of FIG. 6 that will be described later, if a phase is adjusted so that it is fitted about in the middle between DVD±R/RW and DVD-RAM discs, minimum phase differences with respect to DVD±R/RW and DVD-RAM discs deviate from the reference by greater than about 45°. Thus, it is also impossible to realize tracking since the phase difference is extremely large or small.